1. Field of Invention
This invention relates to a process of regenerating an HF-HNO.sub.3 pickle which contains ZrF.sub.4, wherein ZrF.sub.4 is precipitated as Na.sub.2 ZrF.sub.6, the precipitated Na.sub.2 ZrF.sub.6 is removed, and HF, HNO.sub.3 and, if required, H.sub.2 O are made up.
2. Discussion of Prior Art
Such process has been disclosed in U.S. Pat. No. 4,105,469. In the known process, ZrF.sub.4 is precipitated as Na.sub.2 ZrF.sub.6 from the spent pickle by an addition of crystalline NaF and is removed by filtration. That process avoids the disadvantages of the earlier processes in which the spent pickles were neutralized and disposed of so that their zirconium content was lost. However the known process is still relatively expensive and can be carried out only with difficulty.
Because NaF has only a low solubility in water, the precipitant must be added in crystalline form so that an exact proportioning is difficult, particularly in a continuous regenerating process. An addition of dissolved precipitant would excessively dilute the pickle so that the surplus water would have to be removed after the regeneration proper. Besides, NaF is a rather expensive precipitant.
Morever, the process will be adversely affected by an addition of the precipitant in a proportion which is too high or too low. If the regenerated pickle recycled to the pickling bath contains an excessively high content of residual NaF because the precipitant had been added in an excessively high proportion, a precipitation of Na.sub.2 ZrF.sub.6 will already be initiated in the pickling bath so that the pickling operation will be adversely affected. For reasons of safety, zirconium can be precipitated in the known processes only to a residual content of 3 to 7 grams per liter in the regenerated pickle.
On the other hand, the addition of the precipitant in a proportion which is too low results in the formation of NaZrF.sub.5.H.sub.2 O, which is gellike and can be filtered only with difficulty.
It is an object of the invention to provide a process which is of the kind described first hereinbefore and is less expensive and can be carried out more easily and in which the disadvantages which have been described are avoided.